After having not touched the bluetooth-serial interface I built for over a year, I pulled it out of the drawer recently and found it to be dead–specifically, the BT2S module. Seeing that a replacement was going to be $17.95 plus shipping, I began looking for alternatives. I selected the BT2S specifically because it worked at +5V voltage levels. There are a number of similar-looking modules on eBay that sell for much less, but they all use +3.3V supply and logic levels (search for HC-06 on eBay and you’ll see what I’m talking about). Could my circuit be converted to run at 3.3V?
It turns out the answer is yes. I was able to modify the circuit to operate at 3.3V by replacing the MAX232 chip with a MAX3232, the 78L05 voltage regulator with a 78L33 3.3V regulator, and the BT2S with one of the HC-06 bluetooth slave modules available from Amazon, eBay, and several other sources. No changes to the circuit board are needed. I’ve added details on the changes to the project page.